from the link to the Economist article:

They then subdivided these children into those with psychopathic traits and those without and asked, in each case, whether an individual's twin showed bad behaviour, psychopathy, or both.

Their analysis showed that bad behaviour without psychopathy has relatively little genetic component—less than a third. By contrast, four-fifths of the difference in behaviour between the general population and children with psychopathic traits seems to lie in the genes.

Leeneia, your general critique of the research methodology just does not apply in a meaningful sense to the article you have read. If you look at the differential results I have copied here, for most of your points to apply you would have to make quite unusual assumptions. Why should the exposure to violent media be different for identical twins in comparison to fraternal? Why should identical twins be given more or other drugs than fraternal?

In addition to that, newspaper articles rarely report fully about research articles. You wouldn't judge a music performance from reading one review, would you? The full article will be available soon if you want to judge for yourself:

Viding, E., Blair, R. J. R., Moffitt, T. E., & Plomin, R. (in press). Evidence for substantial genetic risk for psychopathy in 7-year-olds. Journal of Child Psychology and Psychiatry

Wolfgang